Effect of Exposure Cycles on . . . Stability of Commercial Particleboard

Twenty-six particleboard types representing both underlayment and standard board were evaluated for water absorption, thickness swelling, and linear expansion after exposure to high humidity or 24 hours of water soaking. Underlayment and standard grade boards manufactured from Douglas-fir and from other species were included as was 3/8-inch and 3/4-inch Douglas-fir plywood. Specimens were conditioned at 90?F. and 30 percent RH before testing. Parallel water soak tests were run with one set laid flat and the other on edge. One set of humidification specimens was exposed to 65 percent RH at 70?F. for 154 days and a second set exposed at 90?F. with RH cycled between 90 percent and 30 percent. Soaking on edge generally gave higher water absorption than soaking laid flat. Linear expansion was generally not affected by soaking position. The thickness swell-soak position relationship was inconsistent. Moisture absorption and linear expansion of the humidification specimens nearly ceased after 14 days. Thickness swell continued for a longer period. Moisture, thickness, and linear change at 90 percent RH were twice those at 55 per cent. Plywood was more stable but several particleboards approached it at 90 percent RH. Maximum moisture increase, linear expansion, and increase in thickness swell occurred early in the cyclic humidification series. Two-thirds of the boards decreased to less than original length on dehumidification. Thickness on dehumidification increased with each cycle.

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